An electric car and a hybrid car on which both an internal combustion engine and a motor are mounted are examples of an electric-powered vehicle on which a secondary battery is mounted. In order to drive such an electric-powered vehicle with electricity, it is necessary to charge the mounted secondary battery from an external battery charger. When charging the secondary battery, it is generally necessary to perform charging control according to the characteristics unique to the battery.
Moreover, when a secondary battery is used for a long period of time, charging and discharging may be performed repeatedly. When charging and discharging is performed repeatedly, the performance of the secondary battery deteriorates, and depending on the deterioration condition, the secondary battery may generate heat and may catch fire. In order to prevent the user of the secondary battery from having severe damage, when using the secondary battery, it is necessary to monitor the deterioration state and perform charging control according to the characteristics of the secondary battery.
Patent Literature 1 discloses a secondary battery system in which a secondary battery is provided with a function of storing the information unique to the secondary battery. The secondary battery system disclosed in Patent Literature 1 notifies the unique information to an external apparatus. When the external apparatus is a battery charger, the battery charger controls charging according to the unique information of the secondary battery, acquired from the secondary battery system.
Patent Literature 2 discloses a charging system that performs charging according to the charging characteristics of a battery. In the charging system disclosed in Patent Literature 2, rather than notifying the battery charger of the unique information of the secondary battery, a reference charging value such as a voltage value or a current value is calculated by taking the unique information of the secondary battery into consideration, and the reference value is notified to the battery charger. Moreover, the battery charger performs charging control of outputting a voltage or a current according to the reference charging value sent from the battery system.
Moreover, Patent Literature 3 discloses a charging device for secondary batteries. The charging device disclosed in Patent Literature 3 detects a terminal voltage of a secondary battery every predetermined intervals of time as the state of the secondary battery, calculates the latest slope and the average slope of a charging curve that indicates a change of the terminal voltage, and determines whether a charging current will be supplied or stopped.
Non Patent Literature 1 discloses a change of the battery performance with charging and discharging. Non Patent Literature 1 states that repeated charging and discharging leads to decrease battery capacity and change charging and discharging characteristics. Moreover, Non Patent Literature 1 also states that the internal impedance of the secondary battery increases when the battery capacity decreases.